1. Field of the Invention
The invention concerns an x-ray system and a method for tomosynthetic scanning of a subject.
2. Description of the Prior Art
3D imaging methods increasingly assert themselves in medical technology. For example, computed tomography (CT), in which an x-ray tube and its opposite detector orbit a patient as an examination subject in order to generate slice exposures of the patient, is hereby a complicated and expensive method. An entire 3D volume is subsequently reconstructed from the slice exposures.
A distinctly less complicated 3D imaging method is digital tomosynthesis, which is in particular developed in connection with mammography. In contrast to CT, an x-ray source is hereby panned around an examination subject as a center of the movement only within a limited angle range, for example ±20° starting from a middle position. An x-ray detector to receive the x-rays hereby remains essentially stationary, i.e. actually stationary, or it is only slightly tilted, for example in order to follow the rotating x-ray tube at least according to the radiation angle. The production of a plurality of x-ray images from respective different angles within the angle region to be panned is designated as tomosynthetic scanning.
Since tomosynthesis should be an optimally cost-effective and uncomplicated method, distinctly simpler (and therefore less resilient) x-ray tubes are hereby used (for example in comparison to CT). The resolution speed of the x-ray tubes is thus limited, and a 50° tomosynthetic scan (i.e. panning of the x-ray tube in an angle range of 50°) for the generation of the multiple x-ray images or projection exposures from various viewing directions today takes between ten seconds and one minute.
A technique known as the dual energy method is also known, in which the subject is exposed twice from each viewing (irradiation) direction with x-ray radiation of two different energies. It is known to execute either two complete scans with a respective energy (i.e. to switch the energy level of the x-ray tube between the two scans) or to execute a single scan and respectively switch the x-ray tube between the two energies in each angle position (i.e. for each projection exposure). The latter also entails a large stress for the x-ray tube (for example in mechanical terms) since both the beam generator and the pre-filtering (swiveling mechanical filter) are switched between two beam qualities.
The scan speed today is normally limited by the x-ray tube since (for example) detectors with fast readout capability (for example 30 images per sec) are used. However, such high image rates can lead to thermal problems in the x-ray tube, not only in the anode but also in the cathode. The thermal problems arise from the need to transport of the heat away from the anode of the x-ray tube since there less than 1% of the supplied energy is converted into x-ray radiation and more than 99% is converted into heat. In contrast to CT with a focal diameter of 1 mm on the anode, due to the higher resolution mammography operates with a 0.3 mm focus, for example, which results in a significantly higher energy density and therefore distinctly greater thermal problems in the x-ray tube.